If left unrefrigerated, how long will lactose-free milk remain fresh?

Some industry experts caution not to leave milk out of the refrigerator even for the time it takes to prepare, eat, and clear away a meal, although others say that’s conservative.  It’s a good idea to pour the approximate amount you need for a particular meal into a glass or pitcher and put the original container back into the fridge.  Then don’t pour leftovers back into the original carton or bottle.
 
“Off” flavors may develop in a clear plastic container of milk exposed to direct sunlight for as little as a few minutes, and strong fluorescent lights can cause vitamin deterioration in as little as 2 hours, according to Cornell University researchers. 
 
Source(s):
Mary Martin Nordness, registered dietician and National Dairy Council Ambassador
Steven C. Murphy, Senior Extension Associate (Milk Quality Improvement Program)
Department of Food Science, Cornell University